The present invention relates generally to touchpad devices, and more particularly relates to trace layout in touchpads.
Touch sensing devices are well known, and take a number of different forms. Perhaps the best known are resistive-membrane position sensors, which have been used in a number of different applications for many years. Such devices have been used as keyboards, position indicators, and so forth. Other types of touch sensing devices include resistive tablets, surface acoustic wave devices, touch sensors based on strain gages or pressure sensors, and optical sensors.
Yet another touch sensing technology is capacitive sensing, in which the location of a finger (or in some instances another object such as a stylus) over a sensing device is determined by virtue of variations in capacitance under and around the location of the finger. Typical of capacitive touch sensing devices are touch screens and capacitive pads which employ a matrix of row and column electrodes and detect, for example, either the transcapacitance between row and column electrodes or the effective capacitance to virtual ground. Other capacitive techniques are also known. Some touch sensitive devices are known to use interpolation for more precisely identifying the location of a finger or stylus.
One problem with the array of rows and columns is that when a finger or stylus moves across the touchpad, the detected signal will have abrupt changes from one line to the next. For example, the finger or stylus may be entirely on one line, then moves across the space between the lines, then abruptly contacts the second line. One attempt to address this problem is in a touchpad in which the rows have a zigzag shape reassembling a series of triangles, one on top of the other. Thus, as the finger moves across, it may contact a tip of a triangle of the next trace before completely contacting the next trace, and before leaving the first trace. This provides more continuity and less of an abrupt change. Since the amount of signal detected is proportionate to the amount of the trace covered by the finger, this is initially a small amount as a tip of an extending triangle is contacted, and increases as the finger gets closer to and eventually covers the trace. A disadvantage of such a design is the zigzag pattern will still not provide a smooth signal depending on the direction of the finger.
For touchpads which determine a capacitance between the matrix and ground, the lower matrix, whether rows or columns, have a disadvantage since they are farther from the finger, and thus have less capacitance coupled into them. This will result in a smaller signal in one direction. One way of addressing this is to include "floating" pads in the upper array. Thus, a series of conductive regions which are not connected to anything are mounted above the corresponding traces of the lower layer to couple in more capacitance from the finger.
It would be desirable to have a trace pattern which is more effective in removing discontinuities.